A technique commonly employed for tempering glass sheets, especially when a series of glass sheets are to be sequentially bend and then tempered, is to vertically hand each glass sheet from tongs which grip the upper marginal edge portion of each glass sheet and to thus convey each glass sheet through heating, bending, and tempering steps. The heating step entails suspending the glass sheets within a heating chamber until the temperature of the glass approaches its softening point, and then each sheet in series is conveyed out of the heating chamber and into a bending station. A typical vertical bending operation is disclosed in U.S. Pat. No. 3,367,764 to S. L. Seymour, wherein a heat-softened glass sheet is bent between a pair of complementary, horizontally reciprocated bending molds. After bending, with the glass sheet still at an elevated temperature sufficient for tempering, the glass sheet is conveyed into a quenching station where it is rapidly cooled by blasts of tempering medium so as to establish compressive stresses in the surface portions of the sheet, thereby strengthening the sheet. The tempering medium is usually air, but as used herein, the term may encompass any fluid capable of cooling a hot glass sheet. Such a process has proved to be an economical, high-speed method for mass producing bent and tempered glass sheets, such as those used for automobile glazing and the like.
One difficulty encountered with tempering glass sheets that are freely hung from tongs is that, when directing the blasts of tempering medium onto the glass sheets, it is usually found to be virtually impossible to precisely duplicate flow conditions on both sides of a curved glass sheet. As a result, sharp side-to-side buffeting of the glass sheet is often induced during quenching. This problem is made more difficult by the fact that it is usually necessary to provide relative motion between the glass sheets and the nozzles applying the tempering medium in order to avoid creating iridescent patterns in the glass due to uneven cooling. Buffeting of the glass sheets impedes uniform application of the tempering medium onto the glass sheets, which in turn leads to imbalanced stresses in the tempered product. Such uneven stresses can result in the tempered glass sheet failing to meet strength specifications and may even cause glass breakage during processng. The problem of buffeting is especially troublesome with thin glass (i.e., glass about 4.5 millimeters or less in thickness), the demand for which has been increasing for use in automobiles. Not only is thin glass lighter in weight and thus more susceptible to buffeting, but also the faster rates of cooling required to temper thin glass entail the use of higher pressure blasts of tempering medium, which in turn increases the amount of buffeting.
Efforts to reduce buffeting in the prior art have included the use of guide wires extending through a quenching apparatus, an example of which may be seen in U.S. Pat. No. 4,006,002 to Hetman. However, such an approach has not been found adequate to stabilize glass sheets in the quench to the extent desired. Moreover, prolonged contact between such guide wires and glass sheets may mar the glass surface in cases where the glass is still sufficiently softened, or may cause lines of distortion in the glass by absorbing heat at a rate different from the rate at which the remainder of the sheet is cooled.
Another prior art approach to limiting the degree of buffeting is disclosed in U.S. Pat. No. 3,824,090 to S. L. Seymour et al. In that arrangement a number of solid rollers are rigidly mounted on brackets attached to the quenching nozzles. However, the relatively large mass of the rollers and brackets serves as a heat sink, which can lead to undesirable non-uniform cooling of the glass. Also, the location of the rollers and brackets within the quenching zone renders adjustments to them very difficult when the apparatus is in use. Furthermore, in the arrangement in the patent, every mode of operation requires the rollers to roll continuously over extended areas of the glass surface.